A visible light sensitive photoinitiator system for the cationic polymerization of typical monomers, e.g. of oxiranes such as cyclohexene oxide, of vinyl ethers such as n-butyl vinyl ether, and of other vinyl monomers such as N-vinylcarbazole, using highly conjugated thiophene derivatives is described. The cationic polymerization of these monomers was initiated at room temperature upon irradiation in the visible region (λ_inc = 480 nm) in CH₂Cl₂ solutions with 4,7-di(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)benzo[1,2,5]thiadiazole (DTDT) or 5,8-bis(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-2,3-di(thiophen-2-yl)quinoxaline (DTDQ) in the presence of diphenyliodonium hexafluorophosphate (Ph₂I⁺PF₆⁻). A feasible mechanism, as correlated with optical absorption and fluorescence spectroscopic measurements, free energy changes (ΔG), and proton scavenging studies, involves formation of exciplex by the absorption of light in the first step. Subsequent electron transfer from excited DTDT or DTDQ to iodonium ion yields radical cations of the thiophene derivatives. The resulting strong Bro̷nsted acid derived from this process catalyzes the cationic polymerization of variety of monomers.